For C++ code, inttypes.h will only define PRId32 if __STDC_FORMAT_MACROS is defined (according to C99, I think). Do so in cpp/var.cc.
For C++ code, inttypes.h will only define PRId32 if __STDC_FORMAT_MACROS is
defined (according to C99, I think). Do so in cpp/var.cc.

BUG=none
TEST=builds everywhere
Review URL: http://codereview.chromium.org/3362016
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